Задать 10 вопросов к тексту as it is known the theory of radio receiving devices was worked out by the Russian scientist V. Siforov. But it is A. Popov who invented and demonstrated the first radio receiving set. Since that time radio devices have been improving and perfecting. The receiver performs the function of converting the current in the receiving antenna into the intelligence contained in the transmission. The main parameters of radio receivers are sensitivity, selectivity and fidelity. Sensitivity is a measure of the receiver's ability to receive weak signals, as it is known that the farther an electromagnetic wave travels, the weaker is its energy. Selectivity is the ability of the receiver to reject undesirable signals. Fidelity is a measure of the receiver's ability to reproduce clearly audio-frequency currents, which are in accordance with the modulation envelope of the received signals. However simple the radio receiver may be, it includes an antenna, an input tuning circuit, a detector and a pair of earphones. The principle of operation of the radio receiver is not very difficult understand. The electromotive force is impressed upon the receiving antenna and produces a current, this current is a reproduction of the current of the transmitting antenna.There are various types of receivers, communication and broadcast receivers being the principal types of them. Communication receivers are used in radio telephone and telegraph service, broadcast receivers finding application for the reception of sound and visual programms. Wherever radio receivers were applied, they must meet an important requirement as reliability in operation.